Gastric adenocarcinoma remains the second most common cause of cancer-related death worldwide. The vast majority of gastric cancer evolves in the stomach in the setting of chronic atrophic gastritis usually in association with Helicobacter pylori infection. While the role of H. pylori as the proximate cause of gastric carcinogenesis is well established, the cellular basis of lineage changes that lead to development of preneoplastic metaplasia and progression to cancer remain unclear. The normal gastric fundic mucosa is assembled from a heterogeneous group of epithelial lineages responsible for the normal secretion of mucins, pepsinogen and HCl. Chronic injury associated with H. pylori infection leads to prominent changes in the composition of the gastric epithelia, with loss of parietal cells (oxyntic atrophy), expansion of surface cells (foveolar hyperplasia) and mucous cell metaplasia. Two metaplastic lineages are now acknowledged in the setting of oxyntic atrophy in humans: intestinal metaplasia (characterized by the presence of intestinal goblet cells in the gastric mucosa) and Spasmolytic Polypeptide Expressing Metaplasia (SPEM; characterized by presence of antral type mucous cells in the body of the stomach). However, Helicobacter infection in mice leads only to SPEM. Over the past 15 years, we have investigated the factors that lead to the development of SPEM in the face of oxyntic atrophy. Using lineage-mapping studies in mice, we have demonstrated that SPEM arises, not from professional progenitor cells, but from transdifferentiation of mature Mist1-expressing chief cells into mucous cell metaplasia. All of these results support the hypotheses that loss of parietal cells from the gastric fundic mucosa induces the development of SPEM from transdifferentiation of chief cells. Since SPEM appears to be the initial pre-cancerous metaplastic response to oxyntic atrophy, it is critical to understand how transdifferentiation of chief cells leads to the emergence of SPEM as the central initial event required for the development of dysplasia and neoplasia in the stomach. We will therefore continue our studies of the origin of metaplasia through the prosecution of three specific aims: First, we will examine the plasticity of chief cell transdifferentiation into metaplasia. Sice our present studies suggest that the ability to transdifferentiate may be a general property of chief cells, we will evaluate whether transdifferentiation induced by oxyntic atrophy is a polyclonal process. Because induction of SPEM is reversible in acute oxyntic atrophy, we will use lineage-mapping studies to evaluate whether SPEM transdifferentiation is directly reversible back into chief cells. Second, we will evaluate the role of CD44variant in the initiation of SPEM. Since we have found that up-regulation of CD44variant marks the earliest point yet identified in the process of transdifferentiation, we will seek to identify critical gene transcription changes attendant with CD44variant up- regulation in chief cells. Further, we will examine whether inhibition of the xCT cystine-glutamate antiporter, a target for activation by CD44variant, can alter the induction or progression of SPEM. Third, we will examine the role of miRNAs in the induction of metaplastic phenotypes by determining whether targeted loss of Dicer expression in chief cells alters the induction of SPEM. All of these studies will help identify fundamental mechanisms involved in chief cell transdifferentiation and may lead to insights in how pre-neoplastic metaplastic lineage changes can be prevented or reversed.